1. Field of the Invention
This invention relates to a fan cylinder assembly constructed from an initially flat rectangular sheet of material and a flat support panel of a cooling tower which panel functions as a guide during forming of the sheet to an exact cylindrical configuration for surrounding a fan in closely adjacent relation to the tip of each fan blade. The cylinder is formed in such a fashion that bending stresses imposed on the sheet are minimized and do not exceed the nominal yield strength for at least 80% of the material in directions along the thickness of the same. In one embodiment of the invention, an annular member presenting an inner convex surface is secured to one side of the support panel opposite the fan cylinder to provide a reduced turbulence, eased inlet for air drawn into the fan cylinder.
2. Description of the Prior Art
Factory assembled cooling towers are characterized as practical, compact units which may be readily installed to provide an air conditioning duty typically within the range of 5 tons to 175 tons of cooling capacity. Cooling towers of this type customarily include an outer housing having an air inlet and an air outlet and supporting a quantity of tower fill between the inlet and outlet. Water to be cooled is distributed to an upper face of the fill and descends by gravity through the fill toward a lower collection basin, while air drawn through the housing and the fill by a fan thermally interacts with the descending water for cooling of the same.
Over the years, continuing efforts have been made toward optimizing the efficiency of cooling towers. One aspect of tower efficiency is related to the motor driven fan and the aerodynamic profile of the fans blades as well as the configuration of the tower housing or structure which defines the path through which the air is directed.
Fans for cooling towers are oftentimes located immediately adjacent and outside the regions of the tower housing containing the fill assembly, and the fan is surrounded by a fan cylinder which is concentrically mounted about a circular opening of a flat panel or support of the housing. It has been found that the efficiency of fan operation can be increased by minimizing the clearance between the outer tips of the fan blades and the inner surface of the fan cylinder. As a consequence, it is important to ensure that the cross-section of the fan cylinder adjacent the blades in a direction transverse to the axis of fan rotation is of an exact circular configuration which does not deform under stress or vibration over extended periods of tower operation to a non-circular configuration, since such deformation could otherwise lead to potentially damaging contact between the tip of the blades and the fan cylinder.
In some instances, fan cylinders comprised of an initially flat sheet of metallic material are "spun" to a configuration including a cylindrical portion surrounding the fan and a gently curved, outwardly flared portion which is mounted on a flat support panel of the tower housing. Cylinders of this type are spun on a forming machine by stretching the sheet metal past its yield strength in portions surrounding a circular hole in the metal until the sheet deflects laterally around the hole and permanently deforms to the desired, somewhat cylindrical configuration. However, such a forming process is costly and must be undertaken with care to ensure that the sheet does not tear or otherwise fail due to stress during the forming operation.
Occasionally, sheet metal is formed to a somewhat cylindrical configuration by a bending apparatus having a pair of spaced, side-by-side, fixed axis rollers and a third roller which is parallel to the two fixed axis rollers and which is selectively shiftable in a reference plane perpendicular to a plane extending through the axis of rotation of the other two rollers. Sheet metal is introduced to the bending apparatus along a path wherein one side of the sheet contacts the pair of fixed axis rollers and the third roller contacts the other side of the sheet. The sheet material is permanently "set" to an arcuate configuration by moving the third roller toward the space between the pair of rollers to a position wherein the tensile and compressive stresses imposed on the sheet are sufficient to overcome the memory of the same. Advancement of the sheet along the path stresses substantially the entire area of the metal past its yield point and deforms the sheet to produce a tubular shape.
Unfortunately, the above mentioned type of bending apparatus does not enable the user to precisely form the sheet matal to an exact cylindrical configuration within relatively small tolerances. One factor influencing the final configuration of the sheet metal is the exact, actual yield strength of the metal which varies from batch to batch and can also vary along the length of the same piece of material regardless of the fact that the sheet metal goods are often designated as having a nominal yield strength.
During use of the three roller bending apparatus, one side of the sheet experiences tensile stress and the opposite side experiences compressive stress, and to overcome the memory of the resilient sheet, the majority of the sheet is exposed to stress which exceeds the yield strength of the same. However, due to the aforementioned variations of actual yield strength portions of the sheet yield to a greater extent than other portions of the sheet even when all of the bending rollers are held in fixed positions and thus the exact radius of curvature of the formed sheet varies from portion to portion. As a consequence, the sheet must be shaped to a tubular dimension larger than the diameter of the swept blade path to ensure that the tips of the blades remain spaced from the cylinder at all times. The necessary, relatively large clearance between the blade tips and the inner surfaces of such fan cylinders in turn substantially reduces the efficiency of the fan and thereby the overall efficiency of the tower.
Moreover, fan cylinders which are of a substantially cylindrical configuration as opposed to the spun cylinders having an outwardly flared inlet region are known to cause turbulence in the air flow and separation of the air from the cylinder walls particularly adjacent the area where the inlet end of the cylinder fixedly abuts a flat panel of the cooling tower surrounding the circular outlet. It would be desirable, therefore, to provide a simple and economical means for reducing air turbulence and separation in towers having a fan cylinder of a substantially cylindrical configuration.